A conventional image processing device implements bold processing by adding image data about an image in which an original image is shifted on a dot basis to the original image data.
FIG. 20 is an explanatory drawing showing bold processing with superimposing.
When performing the bold processing, the conventional image processing device typically uses a method of shifting image data in the form of bit map in a lateral direction or in a longitudinal direction, as shown in FIG. 20, and then superimposing those image data to print out an image.
An example which is an extension of this method using superimposing is disclosed by, for example, the following patent reference 1.
In the method currently disclosed by the following patent reference 1, bold processing is performed on the image and printing is then performed after the one-pixel-width perimeter of the image is fattened through digital image processing.
However, because simple fattening of the one-pixel-width perimeter by one dot causes a difference in the balance of characters between before and after the bold processing, the one-pixel-width perimeter is fattened through the digital image processing in such a way that the balance of characters can be achieved between before and after the bold processing.
In both of the method using superimposing and the method disclosed by above-mentioned patent reference 1, how to bold a character spatially through image processing is a key factor, and what is necessary after holding a character spatially through image processing is basically to simply print out the character using a printer.
Because such a printer is generally a dot impact printer which outputs monochrome binary values, it can be assumed to simply print a dot or not from the viewpoint of a micro level at the time of printing.
Recently, multiple gradation printers which can express a ternary or higher value have been produced commercially, and there have been increasing cases of, when producing a display output, displaying a character image using a multiple gradation display. Taking these recent trends into consideration, it is necessary to assume, as an output device, a multiple gradation of three or more steps.
Hereafter, a case of outputting a character image with a multiple gradation in a method with superimposing which is the most fundamental method of bold processing will be considered as follows. In this case, for the sake of simplicity, assuming that a character image has only one dimension, and when the density value of a pixel at a position x is expressed as P(x) and the density value of a pixel preceding the pixel at the position x by one pixel is expressed as P(x−1), and P (x)=0 shows white and P(x)=Pmax (a maximum density value) shows black, the density value P′(x) of the pixel which is bold-processed is shown by the following expression (1).P′(x)=P(x)+P(x−1)when P′(x)>=Pmax,P′(x)=Pmax  (1)
FIG. 21 is a configuration diagram showing a conventional image processing device. This conventional image processing device carries out bold processing according to the above-mentioned expression (1) to calculate the bold-processed density value P′(x).
In FIG. 21, in order to acquire the density value P(x−1) of the pixel preceding the pixel at the position x by one pixel, a delay means 2101 delays the pixel signal by one pixel, an addition means 2102 adds the density value P(x) of the pixel of the inputted pixel signal and the density value P(x−1) of the pixel preceding the pixel at the position x by one pixel, and a limiter means 2103 limits the addition result of the addition means 2102 to a value equal to or smaller than the maximum density value Pmax, as shown in the expression (1).
In the conventional bold processing with superimposing, and the conventional bold processing of fattening a perimeter portion of a character image, the bold-processed pixel is uniquely determined from the inputted pixel signal according to the expression (1).
In this case, in order to adjust the weight of a character, there's no other choice but to increase the number of adjacent pixels to be referred to make spatial expansion of the character, though when outputting the character image to a multiple gradation printer or a multiple gradation display, it is possible to adjust the weight of the character in appearance using the multiple gradation.
However, because the pixel value is uniquely determined according to the method shown by the expression (1), as mentioned above, there arises a problem of being unable to perform any adjustment of the weight of a character in spite of using an output device which can originally perform fine weight control on the character by controlling the gradation.
Another problem is that a character deformation easily occurs in a case of using an output device having low resolution, such as a display, and in a case of displaying a character font having a small number of points or a character font having a large number of pixels.
FIG. 22 is an explanatory drawing showing a character deformation in the bold processing.
Especially, FIG. 22(a) shows the density value P(x) of a pixel at a position x of a pixel signal inputted, FIG. 22(b) shows the density value P(x) of the pixel and the density value P(x−1) of a pixel preceding the pixel at the position x by one pixel, and FIG. 22(c) shows the result of the addition of the density value P(x) and the density value P(x−1), which is obtained by the addition means 2102.
FIG. 22(d) shows the result of a limiter process which is performed on the addition result shown in FIG. 22(c) by the limiter means 2103. Because the density value P′(x) exceeds the maximum density value Pmax, the density value is limited to the maximum density value Pmax.
A portion shown by a mark ※ of FIG. 22(a) is fully deformed to be black-colored in FIG. 22(d), as can also be seen from FIG. 22(d).
FIGS. 22(e) to 22(g) show an example of two gradation data. FIG. 22(e) shows the density value P(x) of the pixel at the position x of the pixel signal, FIG. 22(f) shows the density value P(x−1) of the pixel preceding the pixel at the position x by one pixel along with the density value P(x) of the pixel at the position x, and FIG. 22(g) shows the density value P′(x) which is limiter-processed.
It is clear also from this figure that in the case of a two gradation image, a white-colored dot is deformed certainly.
Furthermore, a method of, when a character read by a scanner or the like looks patchy, bolding the character to improve its quality is disclosed by, for example, the following patent references 2.
The following patent reference 2 discloses a method of selecting multiple gradation data having a larger density from among certain multiple gradation data of interest and adjacent multiple gradation data adjacent to this multiple gradation data, and defining the multiple gradation data as noticed picture element data.
While this method provides an advantage of being able to suppress character deformations, but because the method has only the process of selecting data having a maximum density, the pixel value is determined uniquely.
Therefore, there is no other choice but to increase the number of reference pixels in the weight adjustment, like in the case of using the above-mentioned method, and there arises a problem of being unable to perform finer weight adjustment by using the gradation.
Especially, when displaying a two gradation font or two gradation character image data on a multiple gradation display having low resolution after performing bold processing on the font or character image data, because the font or character image data are displayed just in the form of two gradation data in spite of being able to display a high-quality character having improved visibility by using the gradation which the display originally has, there arises a problem of being unable to display a high quality character suitable for a gradation display and a multiple gradation printer.    [Patent reference 1] JP, 3-252696,A (see from the 14th line in the upper left column at page 2 to the 7th line in the upper right column at the same page, and FIGS. 1 and 2)    [Patent reference 2] JP, 6-152932,A
A problem with conventional image processing devices which are constructed as mentioned above is that when outputting a character image to a multiple gradation printer or a multiple gradation display, they cannot perform finer weight adjustment using its gradation, and therefore cannot display a high quality character suitable for the multiple gradation display or the multiple gradation printer.
A further problem is that because, especially when outputting a character image to a display having low resolution or the like, conventional image processing devices cannot carry out finer weight adjustment by using the gradation, a character deformation easily occurs in the character image.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an image processing device and an image processing program capable of displaying a higher-quality character by carrying out finer weight adjustment of the character by using the gradation of a multiple gradation display or a multiple gradation printer, and being able to prevent a character deformation from occurring.